Prostheses, tools and methods for replacement of natural facet joints with artificial facet joint surfaces

ABSTRACT

Cephalad and caudal vertebral facet joint prostheses and methods of use are provided. The cephalad prostheses are adapted and configured to be attached to a lamina portion of a vertebra without blocking a pedicle portion of the cephalad vertebra.

CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.10/973,834, filed Oct. 25, 2004, and entitled “Prosthesis, Tools andMethods for Replacement of Natural Facet Joints with Artificial FacetJoint Surfaces,” which is a continuation-in-part of U.S. patentapplication Ser. No. 10/438,294, filed May 14, 2003, and entitled“Prosthesis, Tools and Methods for Replacement of Natural Facet Jointswith Artificial Facet Joint Surfaces,” and further claims the benefit ofProvisional Patent Application Ser. No. 60/567,933, filed May 3, 2004,and entitled “Spinal Prosthesis for Facet Joint Replacement,” all ofwhich are incorporated herein by reference.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

BACKGROUND OF THE INVENTION

I. Vertebral Anatomy

As FIG. 1 shows, the human spinal column 10 is comprised of a series ofthirty-three stacked vertebrae 12 divided into five regions. Thecervical region includes seven vertebrae 12, known as C1-C7. Thethoracic region includes twelve vertebrae 12, known as T1-T12. Thelumbar region contains five vertebrae 12, known as L1-L5. The sacralregion is comprised of five vertebrae 12, known as S1-S5. The coccygealregion contains four vertebrae 12, known as Co1-Co4.

FIG. 2 shows a normal human lumbar vertebra 12. Although the lumbarvertebrae 12 vary somewhat according to location, they share manyfeatures common to most vertebrae 12. Each vertebra 12 includes avertebral body 14 and posterior elements as follows:

Two short extensions/protrusions of bone, the pedicles 16, extendbackward from each side of the vertebral body 14 to form a vertebralarch 18. At the posterior end of each pedicle 16 the vertebral arch 18flares out into broad plates of bone known as the laminae 20. Thelaminae 20 join to form a spinous process 22. The spinous process 22serves for muscle and ligamentous attachment. A smooth transition fromthe pedicles 16 into the laminae 20 is interrupted by the formation of aseries of processes.

Two transverse processes 24 thrust out laterally on each side from thejunction of the pedicle 16 with the lamina 20. The transverse processes24 serve as levers for the attachment of muscles to the vertebrae 12.Four articular processes, two superior 26 and two inferior 28, also risefrom the junctions of the pedicles 16 and the laminae 20. The superiorarticular processes 26 are sharp oval plates of bone rising upward oneach side from the union of the pedicle 16 with the lamina 20. Theinferior processes 28 are oval plates of bone that extend in an inferiordirection on each side.

The superior and inferior articular processes 26 and 28 each have anatural bony structure known as a facet. The superior articular facet 30faces upward or superiorly, while the inferior articular facet 31 facesdownward. As FIG. 3 shows, when adjacent (i.e., cephalad and caudal)vertebrae 12 are aligned, the facets 30 and 31, capped with a smootharticular cartilage, interface to form a facet joint 32, also known as azygapophysial joint.

The facet joint 32 is composed of a superior facet and an inferiorfacet. The superior facet is formed by the vertebral level below thejoint 32, and the inferior facet is formed by the vertebral level abovethe joint 32. For example, in the L4-L5 facet joint, the superior facetof the joint is formed by bony structure on the L-5 vertebra (e.g., asuperior articular surface and supporting bone on the L-5 vertebra), andthe inferior facet of the joint is formed by bony structure on the L-4vertebra (e.g., an inferior articular surface and supporting bone on theL-4 vertebra).

As also shown in FIG. 3, an intervertebral disc 34 between each pair ofvertebrae 12 permits relative movement between vertebrae 12. Thus, thestructure and alignment of the vertebrae 12 permit a range of movementof the vertebrae 12 relative to each other.

II. Facet Joint Dysfunction

Back pain, particularly in the “small of the back”, or lumbosacral(L4-S1) region, is a common ailment. In many cases, the pain severelylimits a person's functional ability and quality of life. Such pain canresult from a variety of spinal pathologies.

Through disease or injury, the laminae, spinous process, articularprocesses, or facets of one or more vertebrae can become damaged, suchthat the vertebrae no longer articulate or properly align with eachother. This can result in an undesired anatomy, pain or discomfort, andloss of mobility.

For example, the vertebral facet joints can be damaged by eithertraumatic injury or by various disease processes. These diseaseprocesses include osteoarthritis, ankylosing spondylolysis, anddegenerative spondylolisthesis. The damage to the facet joints oftenresults in pressure on nerves, also called a “pinched” nerve, or nervecompression or impingement. The result is pain, neuropathy, misalignedanatomy, and a corresponding loss of mobility. Pressure on nerves canalso occur without facet joint pathology, e.g., a herniated disc, due tounwanted bone growth, or as a result of thickening of the soft tissuesof the spinal canal, e.g., Arachnoiditis.

One type of conventional treatment of facet joint pathology is spinalstabilization, also known as intervertebral stabilization.Intervertebral stabilization prevents relative motion between thevertebrae. By preventing movement, pain is desirably reduced.Stabilization can be accomplished by various methods.

One method of stabilization is posterior spinal fusion. Another methodof stabilization is anterior spinal fusion, fixation of any number ofvertebrae to stabilize and prevent movement of the vertebrae.

Another type of conventional treatment is decompressive laminectomy.This procedure involves excision of the laminae to relieve compressionof nerves.

These traditional treatments are subject to a variety of limitations andvarying success rates. Furthermore, none of the described treatmentsputs the spine in proper alignment or return the spine to a desiredanatomy. In addition, stabilization techniques, by holding the vertebraein a fixed position, permanently limit the relative motion of thevertebrae, altering spine biomechanics.

FIELD OF THE INVENTION

This invention relates to prostheses for treating various types ofspinal pathologies, as well as to methods of treating spinalpathologies.

SUMMARY OF THE INVENTION

There is a need for prostheses, installation tools, and methods thatovercome the problems and disadvantages associated with currentstrategies and designs in various treatments for spine pathologies.

The invention provides prostheses, installation tools, and methodsdesigned to replace natural facet joints at virtually all spinal levelsincluding L1-L2, L2-L3, L3-L4, L4-L5, L5-S1, T11-T12, and T12-L1. Theprostheses, installation tools, and methods can restore a desiredanatomy to a spine and give back to an individual a desired range ofrelative vertebral motion. The prostheses, installation tools, andmethods also can lessen or alleviate spinal pain by relieving the sourceof nerve compression or impingement, restoring spinal alignment and/orallowing for partial and/or total immobilization and/or fusion oftreated levels.

For the sake of description, the prostheses that embody features of theinvention will be called either “cephalad” or “caudal” with relation tothe portion of a given natural facet joint they replace. As previouslydescribed, a given natural facet joint has a superior facet and aninferior facet. In anatomical terms, the superior facet of the joint isformed by the vertebral level below the joint (which can thus be calledthe caudal portion of the facet joint, i.e., because it is nearer thefeet). The inferior facet of the joint is formed by the vertebral levelabove the joint (which can thus be called the cephalad portion of thefacet joint, i.e., because it is nearer the head). Thus, a prosthesisthat, in use, replaces the caudal portion of a facet joint (i.e., thesuperior facet of the caudal vertebral body) will be called a “caudal”prosthesis. Likewise, a prosthesis that, in use, replaces the cephaladportion of a facet joint (i.e., the inferior facet of the cephaladvertebral body) will be called a “cephalad” prosthesis.

One aspect of the invention provides a cephalad facet joint prosthesisto replace a cephalad portion of a natural facet joint (e.g., aninferior articular surface and its supporting bone structure on theposterior elements of the vertebra) in the posterior elements of avertebra. According to this aspect of the invention, the prosthesisincludes an artificial facet joint element adapted and configured toreplace a cephalad portion of the natural facet joint and a fixationelement extending from the artificial facet joint element, the fixationelement being adapted and configured to be inserted through a laminaportion of a vertebra to affix the artificial facet joint element to thevertebra, preferably without blocking access to a pedicle portion of thevertebra. The fixation element may also extend into and/or through asecond lamina portion of the vertebra, such as by traversing the midlineof the vertebra through or adjacent to the spinous process. In oneembodiment, after installation the cephalad bearing element is disposedbetween a caudal facet joint bearing surface and a portion of thevertebra, such as a lamina portion.

This aspect of the invention also provides a method of implanting anartificial cephalad facet joint prosthesis on a vertebra and/or theposterior elements of a vertebra. According to this method, a fixationelement is inserted through a lamina portion of the vertebra, and acephalad facet joint bearing surface is placed in a position to form acephalad portion of a facet joint. An artificial facet joint element isattached to a distal end of the fixation element either after or priorto insertion of the fixation element. The fixation element preferablydoes not block anterior, posterior and/or lateral access to a pedicleportion of the vertebra. The fixation element may also extend through asecond lamina portion of the vertebra, such as by traversing the midlineof the vertebra through or adjacent to the spinous process. In oneembodiment, the placing step includes disposing the artificial facetjoint bearing surface between a caudal facet joint bearing surface and aportion of the vertebra, such as a lamina portion. The method may alsoinclude the steps of using a guide to define an insertion path for thefixation element, forming a passage through the lamina corresponding tothe insertion path, and/or prepping the surface of the treated vertebrallevels to accept the cephalad and/or caudal components.

Another aspect of the invention provides a prosthesis to replace acephalad portion of a natural facet joint on a vertebra. In this aspectof the invention the prosthesis includes an artificial facet jointelement adapted and configured to replace a cephalad portion of thenatural facet joint; and a fixation element adapted and configured toaffix the artificial facet joint element to the vertebra withoutblocking access to a pedicle portion of the vertebra. In one embodiment,after installation the cephalad bearing element is disposed between acaudal facet joint bearing surface (either the natural caudal jointsurface or an artificial caudal joint surface) and a portion of thevertebra, such as a lamina portion.

This aspect of the invention also provides a method for implanting acephalad facet joint prosthesis to replace a removed cephalad portion ofa natural facet joint on a vertebra. The method includes the steps ofaligning the cephalad facet joint prosthesis with a caudal facet jointbearing surface; and attaching the cephalad facet joint prosthesis tothe vertebra without blocking a pedicle portion of the vertebra. Theattaching step of the method may also include disposing the cephaladfacet joint prosthesis between the caudal facet joint bearing surfaceand a portion of the vertebra. The attaching step may also include thestep of inserting a fixation element through a portion of the vertebra,such as the lamina. In this case, the method may include the steps ofdefining an insertion path in the vertebra prior to the inserting stepand forming a passage in the vertebra corresponding to the insertionpath. A guide may be used to direct the location and orientation of theinsertion path.

Another aspect of the invention provides a facet joint prosthesis toreplace, on a vertebra, a caudal portion of a natural facet joint (e.g.,a superior articular surface and supporting bone structure on thevertebra). The prosthesis includes an artificial facet joint elementwith a vertebra contacting surface and a caudal bearing surface, thecaudal bearing surface being adapted and configured to replace a caudalportion of a natural facet joint and, in various embodiments, to besubstantially entirely posterior of a contact portion of the vertebrawhen the vertebra contacting surface contacts the contact portion. Theprosthesis also includes a fixation element extending from theartificial facet joint element, the fixation element being adapted andconfigured to be inserted into the vertebra to affix the prosthesis tothe vertebra.

Another aspect of the invention provides a prosthesis for replacing acephalad portion and a caudal portion of a natural facet joint ofcephalad and caudal vertebrae of a spine motion segment. The prosthesisincludes an artificial cephalad facet joint element adapted andconfigured to replace a cephalad portion of the natural facet joint, theartificial cephalad facet joint element having a cephalad bearingsurface; a cephalad fixation element, the cephalad fixation elementbeing adapted and configured to be inserted through a lamina portion ofa vertebra to affix the artificial cephalad facet joint element to thecephalad vertebra; and an artificial caudal facet joint element adaptedand configured to replace a caudal portion of the natural facet joint,the artificial caudal facet joint element including a caudal bearingsurface adapted and configured to mate with the cephalad bearingsurface.

Yet another aspect of the invention provides a method for implanting afacet joint prosthesis to replace removed cephalad and caudal portionsof a natural facet joint of cephalad and caudal vertebrae. The methodincludes the steps of: affixing an artificial caudal facet joint elementto the caudal vertebra; inserting a cephalad fixation element through alamina portion of the cephalad vertebra; and placing an artificialcephalad facet joint bearing surface in a position to form a cephaladportion of a facet joint. The method may also include attaching anartificial cephalad facet joint element comprising the cephalad facetjoint bearing surface to an end of the fixation element either prior toor after the inserting step. The method may also include removal ofvarious bone structures (such as one or more facet structures and/orlaminar material) and/or prepping of the bone surfaces. In at least oneembodiment, the fixation element does not block access to a pedicleportion of the cephalad vertebra. The cephalad fixation element may alsoextend through a second lamina portion of the cephalad vertebra, such asby traversing the midline of the cephalad vertebra through or adjacentto the spinous process. The placing step may also include the step ofdisposing the artificial cephalad facet joint bearing surface betweenthe artificial caudal facet joint element and a portion of the cephaladvertebra. An installation fixture may be used to align the caudal andcephalad elements, although the prosthesis may also be installed withoutusing an installation fixture. The method may also include the step ofusing a guide to define an insertion path for the cephalad fixationelement, although the prosthesis may also be installed without using aguide.

Another aspect of the invention provides a prosthesis to replacecephalad and caudal portions of a natural facet joint of cephalad andcaudal vertebrae. The prosthesis may include an artificial cephaladfacet joint element adapted and configured to replace a cephalad portionof the natural facet joint, with the artificial cephalad facet jointelement including a cephalad bearing surface; a cephalad fixationelement adapted and configured to affix the artificial cephalad facetjoint element to the cephalad vertebra without blocking access to apedicle portion of the cephalad vertebra; and an artificial caudal facetjoint element adapted and configured to replace a caudal portion of thenatural facet joint, the artificial caudal facet joint element includinga caudal bearing surface adapted and configured to mate with thecephalad bearing surface. In one embodiment, after installation thecephalad facet joint bearing surface is disposed between a caudal facetjoint bearing surface and a portion of the vertebra, such as a laminaportion. In one embodiment, the cephalad bearing surface and the caudalbearing surface each has a width along its respective transverse axis,with the cephalad bearing surface width being shorter than the caudalbearing surface width. The artificial caudal facet joint element mayalso include a vertebra contacting surface, with the entire caudalbearing surface being adapted and configured to be posterior of acontact portion of the caudal vertebra when the vertebra contactingsurface contacts the contact portion.

This aspect of the invention also includes a method for implanting afacet joint prosthesis to replace removed cephalad and caudal portionsof a natural facet joint of cephalad and caudal vertebrae. The methodincludes the steps of affixing an artificial caudal facet joint elementto the caudal vertebra; and affixing an artificial cephalad facet jointelement to the cephalad vertebra in alignment with the artificial caudalfacet joint element and without blocking access to a pedicle portion ofthe cephalad vertebra. The second affixing step may also include thestep of disposing the artificial cephalad facet joint element betweenthe artificial caudal facet joint element and a portion of the cephaladvertebra. An installation fixture may be used to align the caudal andcephalad element, although the prosthesis may also be installed withoutusing an installation fixture. The method may also include the step ofusing a guide to define an insertion path for the cephalad fixationelement, although the prosthesis may also be installed without using aguide.

Another aspect of the invention includes devices and methods thatminimize the size and extent of the surgical incision(s) required duringthe repair and/or replacement of facet joints. For example, onedisclosed embodiment of a prosthesis for replacing a cephalad facetjoint can potentially be implanted into a targeted vertebral bodythrough a minimally-invasive cannula. This embodiment can be utilized inconjunction with a surgical incision with exposes only the posteriorportion of the targeted facet joint to be replaced. (Alternatively, thisembodiment can be utilized in conjunction with an endoscopic expandingcannula, such as the Atavi Flexposure® Retractor, commercially availablefrom Endius Incorporated of Plainville, Mass.) Desirably, the surgicalsite is prepared—including removal of cephalad/caudal facet materialand/or decompression of affected nerve fibers—and the cephalad andcaudal prosthesis attached and positioned with little or no disruptionto surrounding tissues, including the supra-spinous and/or inter-spinousligaments.

In another disclosed embodiment of the present invention, the caudalcomponent of a facet prosthesis can be secured to the lamina of theinferior vertebral body, while the cephalad component is secured to oneor more pedicles of the superior vertebral body. This configurationfacilitates the secure placement of a facet prosthesis where some or allof the lamina and/or posterior structures of the superior vertebral bodyhave been removed and/or damaged as a result of injury, disease and/orsurgical intervention.

In another disclosed embodiment of the present invention, both thecephalad and caudal components of a facet prosthesis can be secured tothe lamina of their respective vertebral bodies.

In another aspect of the present invention, there is provided a methodfor implanting a spinal prosthesis by forming a passage from a firstside of a lamina or a spinous process completely through to a secondside of the lamina or the spinous process; advancing a distal end of afastening element from the first side to the second side until aproximal stop of the fastening element rests against the first side; andsecuring a bearing prosthesis to the distal end of the fasteningelement. In additional embodiments, the forming step and the advancingstep are performed percutaneously and/or the securing step is performedpercutaneously. In another alternative, the securing step is performedby placing an element between the bearing and the fastening element. Inyet another alternative, the securing step is performed by expanding thefastening element into an opening in the bearing prosthesis. In anotheralternative, a reinforcing structure or material is provided todistribute forces applied to the first side and/or the second side.

In another aspect of the present invention, there is provided a spinalprosthesis having an elongate body having a distal end and a flaredproximal end; a proximal collar adapted to pass over the distal end andto fit against the flared proximal end; and a prosthetic bearing elementforming a part of an articulating process of the spine, the bearingelement having an outer surface and an internal opening adapted to fitover the elongate body distal end. In another alternative embodiment,the elongate body is long enough to pass completely through a lamina ora spinous process. In another embodiment, the elongate body and theproximal collar are adapted to be percutaneously implanted into aportion of the spine. In yet another embodiment, the prosthetic bearingelement is adapted to be percutaneously implanted into a portion of thespine. In another embodiment there is also provided a distal collaradapted to fit over the elongate body distal end proximal to theprosthetic bearing element. In an alternative embodiment, the elongatebody has a non-circular cross section. In yet another embodiment, aportion of the outer surface of the elongate body is covered with a bonein-growth compound.

In another embodiment of the present invention, there is provided aspinal prosthesis having an elongate body having a distal end andproximal end; a prosthetic bearing element adapted to form a part of anarticulating process in the spine, the bearing element having an outersurface and an internal opening adapted to fit over the elongate bodydistal end; and a shaft having a proximal end and a flared distal end isdisposed within the elongate body such that when the shaft advanceswithin the elongate body an outer surface of the elongate body ispressed against a portion of the prosthetic bearing internal opening. Inanother alternative embodiment, the shaft is threaded to engage with athreaded internal portion of the elongate body. In another embodiment,the distal ends of the shaft and the elongate body are adapted to engagewith a drive instrument, fixing the elongate body while allowingrotation of the shaft. In another embodiment, the shaft proximal endfurther comprises a shearable drive section proximal to a drive section.In yet another embodiment, the elongate body further comprising afeature formed on the elongate body outer surface adapted to engage aproximal collar.

Other features and advantages of the inventions are set forth in thefollowing Description and Drawings, as well as in the appended Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral elevation view of a normal human spinal column.

FIG. 2 is a superior view of a normal human lumbar vertebra.

FIG. 3 is a lateral elevation view of a vertebral lumbar facet joint.

FIG. 4 is a posterior view of an artificial facet joint prosthesisinstalled in a patient according to one embodiment of this invention.

FIG. 5 is a left side view of the embodiment of FIG. 4, as installed ina patient.

FIG. 6 is yet another view of the embodiment of FIG. 4, as installed ina patient.

FIG. 7A is a cross-sectional view of a cephalad bearing element andfixation element according to the embodiment of FIG. 4.

FIG. 7B is a posterior view of a pair of artificial cephalad and caudalfacet joint prostheses according to one embodiment of this invention.

FIG. 7C is a top view of a pair of artificial cephalad and caudal facetjoint prostheses in the embodiment of FIG. 7A.

FIG. 7D is a left view of a pair of artificial cephalad and caudal facetjoint prostheses in the embodiment of FIG. 7A.

FIG. 7E is a bottom view of a pair of artificial cephalad and caudalfacet joint prostheses in the embodiment of FIG. 7A.

FIG. 7F is an anterior view of a pair of artificial cephalad and caudalfacet joint prostheses in the embodiment of FIG. 7A.

FIG. 7G is a bottom view of an alternate embodiment of a pair ofartificial cephalad and caudal facet joint prostheses.

FIG. 8A is a perspective view of an installation fixture according toone embodiment of this invention.

FIG. 8B is a top view of the installation fixture of FIG. 8A.

FIG. 8C is a side view of the installation fixture of FIG. 8A.

FIG. 8D is a back view of the installation fixture of FIG. 8A.

FIG. 9 is an exploded view of the installation fixture of FIG. 8 alongwith a pair of caudal facet bearing elements and a pair of cephaladfacet bearing elements according to one embodiment of the invention.

FIGS. 10A-D are views of a guide tool according to one embodiment of theinvention.

FIG. 11 is a posterior view of the installation fixture of FIGS. 8 and 9to which a pair of caudal facet bearing elements and a pair of cephaladbearing elements have been attached and with the caudal bearing elementsattached to the patient.

FIG. 12 is a left side view of the installation fixture and bearingelements of FIG. 11 with the caudal bearing elements attached to thepatient.

FIG. 13 is a perspective view of the installation fixture and bearingelements of FIGS. 11 and 12 showing a guide tool according to oneembodiment of this invention.

FIG. 14 is a perspective view of the installation fixture and bearingelements of FIGS. 11 and 12 showing the use of a drill bit with theguide tool according to one embodiment of this invention.

FIGS. 15A and 15B are a perspective and cross section views,respectively, of a cephalad prosthesis embodiment of the presentinvention.

FIGS. 16A-16D illustrate a method of implanting and securing theprosthesis of FIGS. 15A and 15B.

FIG. 17 is a left side perspective view of a pair of cephalad prosthesisembodiments constructed in accordance with the teachings of the presentinvention implanted into a vertebral body.

FIG. 18 is a right side perspective view of the cephalad prosthesis ofFIG. 17.

FIG. 19 is an elevated right side perspective view of the cephaladprosthesis of FIGS. 17 and 18.

FIG. 20 is an elevated left side perspective view of the cephaladprosthesis of FIG. 17.

FIG. 21 is a cross-sectional view of an alternative embodiment of acephalad prosthesis constructed in accordance with the teachings of thepresent invention.

FIG. 22 is a cross-sectional view of the cephalad prosthesis of FIG. 21with an associated bearing surface element.

FIG. 23 is a cross-sectional view of the cephalad prosthesis of FIG. 21in a targeted vertebral body.

FIG. 24 is a cross-sectional view of the cephalad prosthesis and bearingsurface element of FIG. 22, in a targeted vertebral body before securingthe bearing surface element.

FIG. 25 is a cross-sectional view of the cephalad prosthesis of FIG. 24,showing placement of the bearing surface element in a locked or securedposition.

FIG. 25A is a cross section view of the cephalad prosthesis of FIG. 25with an alternative embodiment of a bearing surface element.

FIG. 26 is a cross-sectional view of the cephalad prosthesis of FIG. 25,after securing the bearing surface element.

FIG. 27 is a cross-sectional view of the cephalad prosthesis of FIG. 26,after removal of the bearing surface element.

FIG. 28 is a cross-sectional view of the cephalad prosthesis of FIG. 26,showing an alternate re-attachment and placement of the bearing surfaceelement.

FIG. 29 is a perspective view of a counter-torque wrench suitable foruse with various embodiments of the present invention.

FIG. 30 is a side plan view of a functional spinal unit.

FIG. 31 is a side plan view of the functional spinal unit of FIG. 30,after undergoing a decompressive laminectomy procedure.

FIG. 32 is a side plan view of the functional spinal unit of FIG. 31,after implantation of one embodiment of a caudal portion of a facetjoint replacement prosthesis.

FIG. 33 is a side plan view of the functional spinal unit of FIG. 32,after implantation of one embodiment of a cephalad portion of a facetjoint replacement prosthesis.

FIG. 34 is a side plan view of the functional spinal unit of FIG. 32,after implantation of another embodiment of a cephalad portion of afacet joint replacement prosthesis.

FIG. 35 is a perspective view of two reinforcing material embodiments ofthe present invention in place with embodiments of the cephaladprosthesis of FIG. 15A.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DETAILED DESCRIPTION OF THE INVENTION

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention that may be embodied inother specific structure. While the preferred embodiment has beendescribed, the details may be changed without departing from theinvention, which is defined by the claims.

FIGS. 4-7 show artificial cephalad and caudal facet joint prostheses 36and 50 (see FIG. 7C) for replacing a natural facet joint according toone aspect of this invention. Cephalad prosthesis 36 has a bearingelement 38 with a bearing surface 40. In this embodiment, bearingsurface 40 has a convex shape. Bearing element 38 may be formed frombiocompatible metals (such as cobalt chromium steel, surgical steels,titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.),ceramics, polyethylene, biocompatible polymers, and other materialsknown in the prosthetic arts, and bearing surface 40 may be formed frombiocompatible metals (such as cobalt chromium steel, surgical steels,titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.),ceramics, polyethylene, biocompatible polymers, and other materialsknown in the prosthetic arts.

Depending on the patient's disease state, the condition of the patient'snatural facet joint—including the facet joint's strength, location andorientation—may not be acceptable and/or may need to be removed toaccess other spinal structures (such as the lamina and/or anintervertebral disc). As shown in FIGS. 4-7, therefore, the naturalcephalad and caudal facet joint surfaces have been removed to enable theinstallation of a prosthetic facet joint without limitations presentedby remaining portions of the natural facet joint.

In one embodiment of the invention, fixation element 42 attachescephalad prosthesis 36 to a vertebra 60 in an orientation and positionthat places bearing surface 40 in approximately the same location as thenatural facet joint surface the prosthesis replaces. The prosthesis mayalso be placed in a location other than the natural facet joint locationwithout departing from the invention, such as by orienting the fixationelement along a different angle, by moving the joint cephalad/caudad,anteriorly/posteriorly, by moving the joint medially or laterally, orany combination thereof.

In the embodiment shown in FIGS. 4-7, fixation element 42 is a screw.Other possible fixation elements include headless screws, stems, posts,corkscrews, wire, staples, adhesives, bone cements, and other materialsknown in the prosthetic arts.

In this embodiment of the invention, the cephalad facet joint prosthesisattaches to a posterior element of the vertebra, such as one or moreportions of the lamina and/or the spinous process. For example, as shownin FIGS. 4-7, fixation element 42 may extend through a lamina portion 62of vertebra 60 at the base of spinous process 64, traversing thevertebra midline as defined by the spinous process 64 and throughanother lamina portion 66. This orientation of the fixation element issimilar to the orientation used to accomplish translaminar facet jointscrew fixation, as known in the art. Other orientations of fixationelement 42 are possible, of course, depending on the dictates of thespecific vertebral anatomy and the desires of the clinician. Forexample, fixation element 42 may extend through only one lamina portion,only through the spinous process, etc.

Unlike other facet joint prostheses that attach to the pedicle, thisembodiment's use of one or more posterior elements of the vertebra toattach the cephalad facet joint prosthesis of this invention does notblock access to the pedicle area, leaving this area free to be used toattach other prostheses or devices. Other embodiments of the inventionmay occupy, block or impede access to the pedicle area, of course,without departing from the scope or spirit of the invention. Inaddition, because of the inherent strength of the lamina (and thesurrounding cortical bone), the cephalad facet joint prosthesis may beaffixed without the use of bone cement, especially when using a boneingrowth surface, trabecular/coated metal or bioactive ceramics.

In the orientation shown in FIGS. 4-6 as well as in some alternativeembodiments, after insertion the fixation element's proximal end 43(preferably formed to mate with a suitable insertion tool) and distalend 44 lie on opposite sides of the lamina. Bearing element 38 attachesto the distal end 44 of fixation element 42 to be disposed between acaudal facet joint bearing surface (either natural or artificial, suchas the artificial caudal facet joint prosthesis described below) and aportion of the vertebra, such as the lamina portion shown in FIGS. 4-6.To attach bearing element 38 to fixation element 42 in the embodimentshown in FIG. 4, a hole 46 in bearing element 38 is formed with a Morsetaper that mates with the distal end 44 of fixation element 42. Othermeans of attaching bearing element 38 to fixation element 42 may beused, of course, such as other Morse or other taper connections, machinescrew threads, NPT screw threads or other known mechanical, physical(welding, etc.) or chemical fastening means. Fixation element 42 may becoated with antimicrobial, antithrombotic, hydroxyapatite,osteoinductive and/or osteoconductive materials to promote bone ingrowthand fixation. Bearing element 38 may be attached to fixation element 42before or after implantation in the patient, depending on the manner ofimplantation and the requirements of the situation.

Prosthesis 36 may be used to form the cephalad portion of a facet jointwith either a natural caudal facet joint portion or an artificial caudalfacet joint prosthesis.

FIGS. 4-7 also show an artificial caudal joint prosthesis 50 forreplacing the superior half of a natural facet joint according to oneaspect of this invention. Caudal prosthesis 50 has a bearing element 52with a bearing surface 54. In this embodiment, bearing surface 54 isconcave (although the surface could be a myriad of shapes, including,but not limited to, convex, rounded, flattened, complex, and/orspherical bearing surfaces). Bearing element 52 may be formed frombiocompatible metals (such as cobalt chromium steel, surgical steels,titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.),ceramics, polyethylene, biocompatible polymers, and other materialsknown in the prosthetic arts, and bearing surface 54 may be formed frombiocompatible metals (such as cobalt chromium steel, surgical steels,titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.),ceramics, polyethylene, biocompatible polymers, and other materialsknown in the prosthetic arts.

In one embodiment, the natural caudal facet surface has been removed,and fixation element 56 attaches prosthesis 50 to a vertebra 70 via apedicle in an orientation and position that places bearing surface 54 inapproximately the same location as the natural facet joint surface theprosthesis replaces. In an alternative embodiment, the bearing surface54 may be placed in a location different than the natural facet jointsurface, either more medial or more lateral, more cephalad or morecaudad, more anterior or more posterior, and/or rotated or displacedfrom the natural anatomical orientation and orientation. For example,the geometry and function of the artificial joints could be designed toallow for greater-than-natural flexibility and/or movement, to accountfor motion missing and/or lost due to disease, injury, aging and/orfusion of the treated and/or other vertebral levels. In addition, inother embodiments the caudal component can be attached to otherlocations in or on the vertebral body in addition to the pedicle or tothe vertebral body alone.

As shown in the embodiment of FIGS. 4-7, fixation element 56 is a screwattached to bearing element 54 via a hole 58 formed in bearing element52 and is inserted into a pedicle portion 72 of vertebra 70. Otherpossible fixation elements include stems, posts, corkscrews, wire,staples, adhesives, clamps, hooks, bone cements, and other materialsknown in the prosthetic arts. The fixation element 56 can also beinserted into the vertebral body (or portions of the lamina or spinousprocess) in addition to or in place of the pedicle. The fixation elementmay comprise some or all of the bearing surface.

In this embodiment, bearing element 52 has a serrated fixation surface57 adapted to contact a contact portion 74 of vertebra 70. This optionalfixation surface 57 helps prevent rotation of the bearing element 52. Invarious embodiments, the fixation surface 57 may be coated with boneingrowth material, and any optional serrations can increase the surfacearea for bony ingrowth (as well as prevent unwanted rotation of theimplant). As shown in FIG. 5, in this embodiment the entire bearingsurface 54 is posterior to surface 57 and contact portion 74.Alternatively, the bearing elements 52 and 38 could incorporate anoff-center peg or protrusion 56 a (See FIG. 7G) which fits into acorresponding hole or opening (not shown) in the pedicle and/or laminaeto inhibit and/or prevent undesired rotation of the element 52. Inanother alternate embodiment, the cephalad and/or caudal componentscould include an auxiliary fastener or clip (not shown) which secures toor around a portion of the vertebral body to inhibit and/or preventrotation and/or displacement of the caudal or cephalad component. In analternate embodiment, one or more of the bearing elements could comprisean artificial or natural (i.e., allograft, autograft, xenograft or otherbone graft material) substance used to resurface the natural and/ordegenerated facet surface.

Prosthesis 50 may be used to form the caudal portion of a facet jointwith either a natural cephalad facet joint portion or an artificialcephalad facet joint prosthesis. Similarly, an artificial cephalad facetjoint portion may be use in conjunction with either an natural orartificial caudal facet joint component.

FIGS. 7A-F show an artificial facet joint prosthesis according to oneembodiment of this invention apart from the vertebrae. As shown,cephalad bearing surface 40 and caudal bearing surface 54 meet to forman artificial facet joint. As seen best in FIG. 7B, the width of caudalbearing surface 54 along its transverse axis is desirably greater thanthe width of cephalad bearing surface 40 along its transverse axis. Thisfeature helps align the cephalad and caudal joints during implant. Inaddition, this feature permits the point of contact between the twobearing surface to change with flexion, extension, left and rightrotation and lateral bending of the patient's spine. If desired, theprosthesis can be designed to mimic the natural motion and flexibilityof the replaced facet joint, or can alternatively be tailored toaccommodate a lesser or greater degree of flexibility and/or motion (toaccommodate damaged tissues such as discs, etc., or to compensate forco-existing limitations on spinal motion such as existing spinaldeformities and/or adjacent fused levels).

The prostheses of FIGS. 4-7 may be implanted without special tools. Oneembodiment of the invention, however, includes an installation fixtureto assist with the implantation procedure. FIGS. 8-14 show installationtools used to implant two artificial facet joints, i.e., two cephaladfacet joint prostheses and two corresponding caudal facet jointprostheses. The invention also includes installation tools forimplanting a single facet joint prosthesis, two caudal facet jointprostheses, two cephalad facet joint prostheses, a caudal and cephaladjoint prosthesis, or any other combination of facet joint prostheses.

As shown in FIGS. 8 and 9, installation fixture 80 has alignmentelements 82 to align the cephalad bearing elements 38 and caudal bearingelements 52. In this embodiment, the alignment elements are two dowelsfor each bearing element. Alignment elements 82 mate with correspondingalignment elements in the bearing elements, such as holes 84 (shown,e.g., in FIG. 7B) formed in cephalad bearing elements 38 and caudalbearing elements 52. Other alignment elements may be used, of course,such as pins, grooves, indentations, etc. Attachment elements such asscrews 86 attach the bearing elements 38 and 52 to the installationfixture via screw holes 88 (shown, e.g., in FIG. 7B) formed in thebearing elements and in installation fixture 80.

When attached to installation fixture 80, cephalad and caudal bearingsurfaces 40 and 54 are in contact and in proper alignment with respectto each other, as shown in FIG. 8. In one embodiment, the cephalad andcaudal bearing surfaces 40 and 54 are desirably “preloaded” and/orpositioned to be in compression/contact when attached to installationfixture 80. In alternative embodiments, the components of the prosthesiscould be implanted while the joint structure is being distracted, orwhile the joint is held together either by the natural tissue or anartificial construct, or combination thereof. These bearing surfaces candesirably be positioned to either (1) come in contact with each otherwhen the distraction is released, or (2) contact each other and becompressively loaded when the distraction is released. To bring thepairs of bearing surfaces in proper alignment with respect to thepatient's vertebrae, the spacing between the pairs of bearing surfacesmight require adjustment in various embodiments. In the embodiment ofFIGS. 8, 9 and 11-14, installation fixture 80 has two bearing supportcomponents 90 and 92 that move in a controlled manner with respect toeach other. Specifically, in this embodiment a threaded shaft 94 extendsbetween support components 90 and 92. Shaft 94 engages bores formed insupport components 90 and 92; one or both of the bores are threaded sothat rotation of shaft 94 causes support components 90 and 92 to movetowards or away from each other. Shaft 94 may be provided with athumbwheel 96 or other actuator for ease of use. One or more guide rods98 may be provided to maintain the alignment of support components 90and 92. Other means of moving the cephalad/caudal bearing elements pairswith respect to each other may be used, such as a guided or unguidedsliding connection between installation fixture elements.

In use, after preparing the implant site by removal of all or a portion(if desired and/or necessary) of existing natural cephalad and caudalfacet joint portions of the cephalad and caudal vertebrae 60 and 70,respectively, of the spine motion segment, bearing elements 38 and 52are attached to installation fixture 80 as described above. The spacingbetween the bearing element pairs is then adjusted using thumbwheel 96to align the fixation holes 58 of caudal bearing elements 52 with theproper fixation screw insertion sites in the pedicle portions of thecaudal vertebra (or other suitable location), thus placing theartificial facet joints in positions corresponding to the position ofnatural facet joints or in any other position desired by the physician,including positions that do not correspond to the position of naturalfacet joints. Passages aligning with holes 58 are formed in thepedicle—or into another part of the caudal vertebra near or adjacent tothe pedicle—using a drill, awl, pedicle probe, or other tool known inthe surgical arts. Fixation screws 56 are then inserted through holes 58into the pedicle or other portion of the caudal vertebra to attach thecaudal bearing elements as well as the entire prosthesis andinstallation fixture to the caudal vertebra 70, as shown in FIGS. 11 and12. Alternatively, self-tapping screws or other caudal fixation elementsmay be used, thereby eliminating the need to pre-form the passages.

Thereafter, the cephalad bearing elements are attached to the cephaladvertebra 60. In one embodiment, an insertion path is first determinedfor each fixation element, then a passage is formed along the insertionpath corresponding to cephalad bearing element holes 46 (e.g., in thelamina at the base of the spinous process and through the lamina on theother side, through only one lamina portion, through the spinousprocess, etc.). Fixation screws 42 can then be inserted through theholes 46 into the passages. Alternatively, self-tapping screws or othercaudal fixation elements may be used, thereby eliminating the need topre-form the passages.

After all four bearing elements have been affixed, the installationfixture 80 may be detached and removed. Installation fixture 80 may beused to implant fewer than four bearing elements, of course.

FIGS. 10, 13 and 14 show a tool that may be used to define the insertionpath (location, orientation, etc.) for the fixation element of the leftcephalad bearing element. For example, the tool may be used to guide theformation of a cephalad bearing element attachment passage for the leftbearing element. A corresponding mirror image tool may be used for theright cephalad bearing element. In alternative embodiments, a singletool may be used for defining the insertion path for both left and rightcephalad bearing elements.

As shown, tool 100 has a handle 102 and an alignment interface (such asdowels 104 in tool 100 and holes 106 in fixture 80) to align the tool inthe proper orientation with respect to installation fixture 80 and acephalad facet joint bearing element. With the caudal and cephaladbearing elements still attached to installation fixture 80 andpreferably with caudal bearing elements already affixed to the caudalvertebra 70, the tool 100 engages installation fixture through thealignment interface as shown in FIGS. 13 and 14. In this position, thetool 100 may be used to define an insertion path for the cephaladfixation elements.

In the embodiment shown in FIGS. 10, 13 and 14, the insertion path guideis a drill guide 108 supported by arms 110 and 112 and is aligned withthe hole 46 in cephalad bearing element 38 by the alignment interfacebetween installation fixture 80 and guide tool 100. In this embodiment,the drill guide 108 is a tube, but other guide elements may be used,such as a guide groove or surface. (Alternatively, the insertion pathguide could be an 11-gage spinal needle, or a cannula sized toaccommodate the cephalad prosthesis components for a minimallyinvasive—MIV—procedure.) A drill bit 114 may be inserted through thedrill guide 108 to form an insertion passage, such as a passage througha lamina portion of the cephalad vertebra. A fixation screw may then beinserted through the passage in the cephalad vertebra and into the Morsetaper connection of the hole 46 (or other type connection, as discussedabove) of the cephalad bearing element 38. As discussed above, thefixation screw may be coated with a bone ingrowth material.Alternatively, a self-tapping screw may be used, thereby removing theneed to pre-form a passage.

In order to determine the length of the passage (especially during aminimally invasive procedure), as well as to prevent over-drilling ofthe passage, the proximal shaft of the drill can include a drill-stop(not shown) to prevent advancement of the drill into the cannula beyonda desired depth. Similarly, the proximal shaft of the drill can includedepth markings which, when the drill exits the passage, can be used todetermine the length of the passage created in the lamina. Desirably,subtracting the length of the cannula (which is known) from the depthmarkings can provide an accurate estimate of the passage length, andthus assists the physician in the choice of the proper size cephaladimplant to fill the passage.

A mirror image tool may then be used to define an insertion path or toform a hole for the right cephalad bearing element, which is thenaffixed to the vertebral body in the same way. The installation fixtureis then removed, such as by unscrewing screws 86.

As mentioned above, in alternative embodiments the guide tool may beused to define a path for a self-tapping screw or other fixation elementthat does not require the use of a drill. In those embodiments, element108 may be used to define a path for the self-tapping screw or otherfixation element. The fixation element path may be through only a singlelamina portion, through the spinous process alone, or any other suitablepath.

In some embodiments, the entire prosthesis other than the bearingsurface may be coated with bone ingrowth material.

FIGS. 15A and 15B depict a side plan and cross-sectional view,respectively, of an alternate embodiment of a cephalad portion of afacet replacement prosthesis constructed in accordance with theteachings of the present invention. In this embodiment, the facetprosthesis 200 comprises a substantially cylindrical body 205 having aproximal end 210, a distal end 215 and an expanded section 250 adjacentthe proximal end 210. In use, a proximal collar 220 is adapted to secureagainst the proximal end, preferably against a portion of the expandedsection 250. A distal collar 225 is secured about the body 205 in aposition proximal to the distal end 215. A ball 230, incorporating oneor more bearing surfaces 235, is positioned at the distal end 215. Theball 230 is an example of a prosthetic bearing element adapted to form apart of an articulating process in the spine. As illustrated, the ball230 has an outer surface and an internal opening adapted to fit over theelongate body distal end. While the dimensions and specific design ofthe ball 230 and collars 220, 225 may secure to the body 205 unaided,the illustrated embodiment of the prosthesis 200 includes a ball shim240 to aid in securing the ball 230 into position on the body 205, and adistal collar shim 245 to aid in securing the distal collar 225 intoposition on the body 205. While the body 205 is illustrated anddescribed having a circular body, it is to be appreciated that othernon-circular shapes, such as for example, triangular, rectangular orother polygonal shape may be used and the components described hereinadapted as needed. Non-circular cross section bodies advantageouslyprovide the prosthesis an anti-rotation capability.

In additional alternative embodiments, features, compounds, or surfacetreatments may be utilized to enhance attachment between the variouscomponents of the prosthesis 200 or between the prosthesis 200 andvertebral bone. For example, portions of the collars 225, 220 thatinteract with the body 205 may be textured or have teeth to promote astronger attachment when joined to the body 205. Similarly, portions ofthe collars 225, 220 that come into contact with the lamina/spinousprocess may also include features or surface textures to promote joiningas well as compounds, for example, bone growth compounds or cements, topromote adhesion between the bone and the collars 220, 225. Similarly,the surfaces of the shims 245, 240 may also include features or surfacetreatments to improve contact and grip between the distal collar 225 andthe ball 230, respectively, as well as the body 205. In much the sameway, the exterior surface of the body 205 may also be adapted to includefeatures, compounds, or surface treatments to improve contact with theshims 245, 240, collars 225, 220, ball 230 and the exposed bone in thepassage 290 (see FIG. 16A).

In one specific embodiment, the prosthesis 200 has an elongatedcylindrical body with a length of 40 mm, a minimum diameter of 4 mm, adistal end that expands to approximately 5 mm, and a ball 230 having ametal sphere with a diameter of approximately 10 mm. The size, lengthand dimensions of the components of prosthesis 200 may vary and beselected based on a number of criteria. Examples of selection criteriainclude the age and sex of the patient, the specific pathology andanatomy of the patient and the specific spinal level where implantationwill occur. The measurement techniques and tools described herein may beused to determine the size, dimensions and placement of a specificprosthesis 200.

A method for implanting the prosthesis 200 will now be described withreference to FIG. 16A-16D. A passage 290 is formed completely throughthe remaining lamina and/or spinous process of the cephalad vertebrabetween a first outer surface of lamina and/or spinous process 285 and asecond outer surface of lamina and/or spinous process 287. The passage290 is adapted to accommodate the prosthesis 200, specifically the body205 (FIG. 16A). In one specific embodiment, the passage 290 is sized toaccept the cylindrical body 205 but not the expanded section 250. Toinstall the prosthesis 200 into a passage 290, a proximal collar 220 isslid over the distal end 215 of the body 205. Note that the proximalcollar 220 is sized and shaped to engage with a portion of the expandedsection 250. Next, the body 205 is advanced distal end 215 first throughthe passage 290 until the proximal collar 220 abuts against the firstouter surface of the lamina and/or spinous process 285 and the expandedsection 250.

As shown in FIG. 16B, once the distal end 215 exits the passage 290beyond the second outer surface of the lamina and/or spinous process287, the distal collar 225 slides over the distal end 215, abuttingagainst the second outer surface of the lamina and/or spinous process287. Next, as shown in FIG. 16C, a distal collar shim 245 is pushedbetween the body 205 and the distal collar 225 to secure the distalcollar 225 in position. Advantageously, the distal collar 225 and theproximal collar 220 are compressed and/or tightened against the outersurfaces of the lamina 285,287, desirably against one or more corticalbone surface(s) of the lamina and/or spinous process.

Next, as shown in FIG. 16D, the ball shim 240 and bearing 230 areadvanced over the distal end 215. Similarly, the ball shim 240 is usedto wedge the bearing 230 into the desired position relative to thedistal end 215. If desired, a prosthesis 200 for replacing the opposingcephalad facet joint on the same vertebral body can be placed in asimilar manner into a corresponding passage 290 through the laminaand/or spinous process resulting in two prosthesis per level asillustrated, for example, in the several views of FIGS. 17-20.

As best shown in FIGS. 17 through 20, a pair of such prosthesis 200 canbe secured through the lamina and/or spinous process of a targetedvertebra to replace a pair of natural cephalad facet structures. Aspreviously noted, the surgical procedure may include, but is not limitedto, the removal and/or resection of one or more sections of the laminaand cephalad/caudad facet joint structure to alleviate nerve compressionand/or spinal stenosis, remove disease or damaged tissues, prepare theintervertebral disk space to receive an artificial diskreplacement/augmentation prosthesis, provide access to spinalstructures, or for other reasons.

In one embodiment of a surgical procedure for implanting the prosthesis200, the targeted facet capsule is initially exposed (the open portion)using standard open surgical techniques. The facet capsule is thenopened and/or removed, and the superior and/or inferior facets areresected and/or removed as necessary (using a surgical cutter orrongeur) during the surgical procedure. If replacement of the caudalfacet section is deemed necessary, a caudal stem and associated caudalbearing can be implanted into the exposed pedicle through the openincision.

Advantageously and in contrast to conventional techniques where both thecephalad and caudal prosthesis are implanted via an open procedure, amajority of the components of the prosthesis 200 can be surgicallyimplanted using minimally-invasive techniques alone or in combinationwith conventional open techniques. For example, all or most of theprosthesis 200 may be delivered through a cannula inserted through asmall incision in the skin. To implant the cephalad implant, thephysician can first create an access path through the skin and softtissue (with a spinal needle and/or K-wire) to the lamina of thetargeted vertebral body. Desirably, non-invasive visualization, such asfluoroscopic or real-time MRI, is used to monitor the advancement of theneedle and avoid damage to tissue structures such as muscles, tendons,ligaments, nerves, veins and/or the spinal cord itself. Once the accesspath has been created, a suitable cannula can be advanced through thetissues to the targeted bone. If necessary, progressively largerdilation catheters (such as the Access™ Dilation Port commerciallyavailable from Spinal Concepts of Austin, Tex.) can be used to introducea cannula having a lumen large enough to accommodate passage of thecephalad implant (i.e., the body 205 and proximal collar 220). Inalternative embodiments, one cannula is positioned and adapted todeliver the body 205 and the proximal collar 220 and another cannula ispositioned and adapted to deliver the remaining prosthesis components.

Once the cannula is in position against the lamina, a drill is advancedthrough the central opening in the cannula and drills into and throughthe targeted portion of the lamina and/or spinous process, creating apassage through the lamina. Desirably, a positioning tool (such as thetool 100 shown in FIGS. 10A-10D) will be used to align the cannulaand/or drill (and thus the passage created) such that the passage isaligned to permit the bearing surface of the cephalad implant to matewith the corresponding caudal bearing surface. In at least oneembodiment, the position of the drill tip can be visually verified (asthe drill tip exits the lamina) through the open incision. In onealternate embodiment, the positioning tool aligns the cannula and/ordrill relative to the caudal bearing surface. In another alternateembodiment, the positioning tool aligns the cannula and/or drillrelative to the upper endplate of the caudal vertebral body, eitherprior, during, or after the initial access through the patient's skinand/or during the surgical procedure.

After creation of the passage 290, the drill (and any alignment frame,if desired) is removed (with the cannula desirably remaining in place inthe patient), and the cylindrical body 205 and associated proximalcollar 220 are advanced through the cannula and into the passage 290 inthe lamina. The proximal collar 220 is desirably seated against the nearsurface of the lamina (i.e., the first outer surface of the lamina orspinous process 285), with the distal end 215 extending out of the farsurface of the lamina into the open incision (i.e., beyond the secondouter surface of the lamina or spinous process 287.) The distal collar225 and shim 245 are then placed on the distal end 215 and tightenedinto position as described above. Desirably, the distal and proximalcollars 225, 220 will compress and bear directly against the far andnear outer surfaces of the lamina and/or spinous process 285, 287, withthe lamina and/or spinous process in between. Once the cephalad implantis secured in its desired position, the cannula can be removed, ifdesired.

Next, using the access provided by the open incision, the ball 230 ispositioned over the distal end 215 into the desired position and securedusing the shim 240 as previously described. Once the prosthesis 200 isimplanted and secured into position, the open surgical site can beclosed in a known manner, and the surgical procedure completed.

If desired, various embodiment of the caudal and/or cephalad componentsdisclosed herein could incorporate non-circular posts or stems foranchoring the devices. Passages to accommodate such constructs could becreated using broachers, reamers, awls, punches or the like. Suchnon-circular stems would desirably reduce and/or prevent unwantedrotation of these components along their longitudinal axis.

FIG. 21 depicts an alternate embodiment of the present inventionconstructed in accordance with the teachings of the present invention.Because many features of this embodiment are similar to componentspreviously-described in connection with other embodiments, likereference numerals will be used to describe similar components. Thecephalad prosthesis 200 a comprises a substantially cylindrical hollowbody 205 a having a proximal end 210 a, a distal end 215 a and alongitudinally-extending bore 207 a there through. The body 205 aexterior has an enlarged outer ridge 217 a, and a wrench-engagementsection 209 a positioned near the proximal end 210 a. The size, shape,and contours of the enlarged outer ridge 217 a are adapted to engagewith the proximal collar 220 a (e.g., FIG. 23). The outer or engagementsurfaces of the ridge 217 a and/or collar 220 a may include features,surface treatment or compounds as described herein or known to those ofordinary skill in the prosthetic arts to improve joining between thosecomponents or adjacent bone.

Returning to FIG. 21, a cylindrical shaft 250 a, extending through thebore 207 a and sized to permit relative movement therein, has a set ofexternal threads 255 a which mate with corresponding internal threads260 a inside the bore 207 a. The shaft 250 a has a proximal drivesection 270 a, a revision drive section 275 a and a notched link 280 athere between. The shaft 250 a includes an enlarged distal plug 265 asized and adapted to deform the distal end 215 a when advancedproximally relative to the body 205 a via engagement of threads 255 a,260 a. FIGS. 22 and 23 illustrate an embodiment of a bearing 235 ahaving an aperture 233 sized to fit over distal end 215 a. Theprosthetic bearing 235 a is an example of a bearing element adapted toform a part of an articulating process in the spine. The bearing 235 ais illustrated adjacent (FIG. 23) and in position over the distal end215 a (FIG. 22, 24) before proximal advancement of the distal plug 265a. When the shaft 250 a is advanced, the plug 265 a deforms the distalend 215 a into pressing contact with the surfaces of the aperture 233locking the bearing 235 a in place (FIG. 25).

To install the cephalad prosthesis 200 a in a targeted vertebral body300 a, a passage 305 a is drilled completely through the lamina and/orspinous process from a first outer surface 385 to a second outer surface387 of a lamina and/or spinous process as previously described (thepassage 305 a is illustrated in FIG. 23). Desirably, the physician willchoose and form the passage 305 a based on the condition of the lamina,the patient's specific anatomy, the physician's knowledge of anatomy,and the desired final location of the cephalad bearing surface relativeto the vertebral body. If desired, the physician can initially place acaudal bearing surface, and then use a jig or other tool to determinethe desired orientation and position of the passage that must be createdto implant a cephalad prosthesis that properly mates with the caudalbearing surface.

Next, after creating the passage 305 a completely through the lamina,the physician will slide a proximal collar 220 a over the distal end 215a up to engagement with the exterior surface of the ridge 217 a. Thebody 205 a is then advanced distal end 215 a first until the proximalcollar 220 a seats against the first outer surface of the lamina and/orspinous process 385 and the distal end 215 a desirably extends out ofthe other end of the passage 305 a (i.e., beyond the second outersurface of the lamina and/or spinous process 387) (FIG. 23). In anembodiment where a combination of an open procedure and minimallyinvasive procedure are advantageously combined as described above withregard to prosthesis 200, the distal end 215 a would extend into theopen surgical space while the first outer surface 385 was accessed andthe passage 305 a was formed using minimally invasive techniques asdescribed herein or known to those of ordinary skill in the surgicalarts.

Next as shown in FIG. 23, a distal collar 225 a is advanced over thebody distal end 215 a until the collar 225 a contacts the second outersurface of the lamina and/or spinous process 387. Optionally, a shim orwedge as described above with shims 240, 245, may be driven between thedistal collar 225 a and the body 205 a to secure the distal collar 225into position. Next, as shown in FIG. 24, the bearing 235 a is advancedsuch that the distal end 215 a is disposed within the aperture 233. Inthe illustrated embodiment, the bearing 235 a is advanced over thedistal end 215 a until the inner surface 310 a of the bearing 235 aabuts against the body distal end 215 a. It is to be appreciated thatthe bearing 235 a may, depending upon a number of factors such aspatient anatomy and relationship to other prosthetic components, engagethe distal end 215 a in a position other than against the inner surface310 a (e.g., FIG. 28). Alternatively as shown in FIG. 25A, an opening233 a may extend completely through a bearing 235 b.

Returning now to FIG. 24, once the bearing 230 a is properly positionedover the distal end 215 a, the physician can utilize a counter-torquewrench 400 (see FIG. 29) having a rotating driver section 420 whichengages the proximal drive section 270 a of the shaft 250 a, and astationary driver section 410 which engages the wrench-engagementsection 209 a of the body 205 a. Desirably, the wrench 400 rotates thedrive section 270 a relative to the wrench-engagement section 209 a withlittle or no movement of the prosthesis relative to the lamina. Thisrotation will desirably draw the enlarged distal plug 265 a further intothe distal end 215 a of the body 205 a, causing the distal end 215 a toexpand outward and wedging (and securing) the bearing 230 a to the body205 a (FIG. 25). In one alternate embodiment, the bearing can alsosecure the distal collar 225 a against the lamina, rather than by usinga shim or wedge to secure the collar 225 a.

Once the bearing 230 a is secured to the body 205 a with apre-determined amount of force, further tightening of the counter-torquewrench will desirably shear the shaft 250 a at the notched section 280a, preventing further rotation and/or over-torqueing of the shaft 250 a(FIG. 26). If desired, the end of the wrench 400 can include a detentsection (or other type of catching or holding mechanism—not shown) whichdesirably contains the sheared section of the shaft 250 a.

If desired, implantation of a similar cephalad prosthesis correspondingto the complimentary facet joint can be accomplished in a like manner.

In the event that revision and/or removal of the cephalad prosthesis isdesired or required, the present embodiment also facilitatesrevision/removal of the cephalad prosthesis. To remove the prosthesis,the counter-torque wrench 400 can be used to rotate the shaft 250 a(relative to the body 205 a) in a direction opposite to the “tighteningdirection”, thereby advancing the shaft 250 a distally and eventually“pushing” the bearing 230 a off of the distal end 215 a of the body 205a (FIG. 27). Desirably, removal of the bearing 230 a will also compressthe distal end 215 a of the body 205 a to some extent, simplifyingremoval of the distal collar 225 a as well as withdrawal of the body 205a proximally through the passage 305 a and ultimately from the treatmentsite (if so desired). In a similar manner, the caudal bearing surface(or both surfaces) could be repaired and/or replaced and/or repositionedif desired and/or necessary (FIG. 28).

The previously-described procedure could similarly be used to replace aworn and/or damaged bearing surface of a previously-implanted cephaladconstruct, without requiring removal and/or revision of the entirecephalad implant. Simply removing and replacing the worn or damagedbearing with a new/undamaged bearing, and retightening of the cephaladimplant, could be accomplished with little or no disruption to thesurrounding tissues. Moreover, such a repair procedure could beaccomplished using a first cannula to access the proximal drive section270 a and associated components, and a second cannula to access thebearing 230 a (to remove the old bearing and introduce a newreplacement).

In alternate embodiments, the bearing surfaces 235, 235 a of thebearings 230, 230 a could be non-spherical (including oval, square,triangular, flattened and/or disk shaped), and could include one or morebearing surfaces 235 (i.e., more than one bearing surface 235 asillustrated in FIG. 15A).

Depending upon the patient's condition and the desired surgical outcome,as well as the surgeon's preference, the present embodiment canfacilitate the repair and replacement/augmentation of the facet jointsin a minimally-invasive, limited-open (or modified-open) and/orfully-open surgical procedure. For example, where facet jointreplacement is deemed necessary, but removal of soft and/or hard tissuesin and/or adjacent the spinal canal is not warranted or desired (such aswhere spinal stenosis and nerve impingement is not a significantconcern), the repair and/or replacement of one or more facet joints canbe accomplished in a least-invasive fashion, using one or more cannulaeto implant the prosthesis and associated distal hardware. Alternatively,where removal of the facet joints and/or lamina is necessitated, such aprocedure can be accomplished through a combination of open, semi-openand/or minimally invasive procedures (which will be referred to hereinas a modified-open or mini-open procedure) to minimize damage and/ordisruption to surrounding soft-tissue structures. In such a procedure,one or more of the facet joint capsules can be exposed through an openincision (to allow easy resection and removal of the facet joint and/orsurrounding anatomical structures), and the cephalad component of thefacet replacement can be delivered through the lamina through a cannulaor other minimally-invasive delivery method.

Another significant advantage attendant with the present embodiment isthat the majority of the cephalad prosthesis is positioned within thelamina, with only limited portions of the implant extending outwardsfrom the vertebral body. This arrangement presents a low-profile to thesurrounding soft tissue structures, desirably resulting in lessinteraction between the prosthesis and the surrounding soft tissues, aswell as less displacement of natural tissues due to the presence of theimplant. Moreover, anchoring the cephalad portion of the prosthesiswithin the lamina and/or spinous process reduces and/or eliminates toopportunity for unwanted contact between the dura and the prosthesis.

Another significant advantage attendant to various disclosed embodimentsresults from the location and attachment method of the cephalad portionof the prosthesis. Because the location, length and orientation of thelaminar passage created by the surgeon is variable (depending upon thepatient's anatomical constraints), a limited variety of implant sizesand/or shapes can accommodate almost any anatomical variation possiblein the patient. For example, a kit including the cephalad implant caninclude cephalad implants having various lengths, including 30, 40, 50and 60 millimeters, to accommodate passages/lamina having differinglengths/thicknesses. Similarly, the depth of the hole 233 in the bearing235 a (to accommodate the distal end of the cephalad component) can vary(by 1 or ½ mm increments, for example) to accommodate anatomicalvariations in the patient. Thus, the present embodiment and implantationmethods reduces the need for a highly-modular and/or configurablecephalad prosthesis. Moreover, the present implant design canaccommodate bones of varying dimensions and/or configurations. Moreover,the solid nature of the component retains its strength and durability.

Another significant advantage of various embodiments described herein isthe use of the lamina and spinous process as the anchor points for thecephalad portion(s) of the prosthesis. By avoiding use of the pediclesof the cephalad vertebral body to anchor the cephalad prosthesis, thepresent embodiment (1) reduces the opportunity for unintended damage toand/or intrusion into the facet joint capsule and facet joint structuresof the cephalad vertebral level being treated, (2) allows for subsequentor concurrent implantation of additional prosthesis into the pedicles ofthe cephalad vertebral body, (3) allows unrestricted access to theintervertebral disk and disk structures in event of the need forconcurrent or subsequent disk treatment, and (4) utilizes the lamina(and thus the pedicles) to support the prosthesis in a more naturalanatomical manner. Desirably, the present embodiment will permit aphysician to “daisy-chain” multiple prosthesis along multiple vertebrallevels, during either a single surgical procedure or during subsequentsurgeries as additional facet joints degrade and/or degenerate.

By anchoring the cephalad prosthesis within the lamina and/or spinousprocess, rather than within the pedicle, the present embodiment moreclosely mimics the natural anatomical position and loading of thecephalad facet joint surface and vertebral bodies. Loads which wouldhave originally been transmitted from the inferior facet joint throughthe lamina and pedicles and into the vertebral body (which would bedirectly conducted through the pedicle and into the vertebral body by apedicle-based cephalad implant anchoring system) are now simplytransferred through the cephalad prosthesis and into the lamina in amore natural anatomical loading manner. Moreover, the use of the laminaas an anchoring point for the implant significantly reduces the forcesexperienced by the bone at the anchor (desirably reducing the tendencyfor the implant to break or loosen over time).

Desirably, the proximal and distal collars can move and/or rotate to alimited degree relative to the cylindrical body of the cephalad implant,such that, when the implant is in position and tightened, the collarswill lie relatively flat against the cortical bone walls of the laminaat either or both the proximal and distal ends of the cephalad implant.If desired, the physician can alter one or both sides of the lamina tomore readily accommodate the proximal and/or distal collars.

Desirably, the cephalad stems will be secured directly to the lamina andwill incorporate a bony-ingrowth surface. Alternatively, the stems canbe secured to the lamina using bone-cement or osteo-conductive orosteo-inductive material. Desirably, any such securing material willresist progressive loosening and fracture propagation typicallyassociated with long-term implantation of orthopedic joint replacements.

In another alternate embodiment, the lamina passages could cross througheach other, with the bodies of the individual cephalad prosthesisconnecting or “linking in one or more manner (either inside the laminaor externally to the lamina, or some combination thereof) to moresecurely “solidify” the fixation and rigidity of the construct. Forexample, one cephalad implant could incorporate a through-hole ofvarying size to accommodate a corresponding distal end of acorresponding implant. Alternatively, the implants could be bridged by alocking collar extending over, under and/or through the spinous process.

In another embodiment, the cephalad implant or portions thereof (i.e.,the body 205, 205 a) can be non-cylindrical, and the correspondinglyshaped passage through the lamina can be created using a chisel,rongeur, broach or “box punch”. Such a non-cylindrical implant woulddesirably resist rotation to a significant degree.

In another alternate embodiments, the cephalad and caudal components ofthe prosthesis can be linked together to reduce and/or eliminaterelative motion between the cephalad and caudal components, and thusbetween the cephalad and caudal vertebral bodies. For example, whereprogressive degeneration of the intervertebral disk and/or vertebralbodies renders the spinal motion segment significantly unstable, orwhere reduction of the relative motion between the vertebral bodies isdesired, the cephalad components can be “capped” or locked to the caudalcomponents. Such linking mechanism could include clamps or wraps (suchas wire ties) which secure the bearing surface within the caudal cup, aswell as adhesives which could secure the components to each other or“fill” a clamp or cup used to “fuse” the articulating surfaces of theimplant.

Many of the features of the present embodiment are designed toaccommodate significant variability in the anatomy of vertebral bodies.For example, the bearing 235 a need not be positioned flush against thedistal end 215 a, but can rather be secured to the body 205 a at variouslocations, thereby incorporating some length variability into the system(FIG. 28). Similarly, the distal collar 225 a and wedge (if used) can besecured to various positions along the body 205 a. Similarly, theproximal collar 220 a can incorporate an offset through-hole (such as a15° offset or a 30° offset from perpendicular) to accommodate evengreater angular variability in the first outer surface of the lamina 385relative to the passage 305 a. Similar variations are possible for thecomponents in embodiments of prosthesis 200.

FIGS. 30 through 34 depict other alternate embodiments of facet jointreplacement prostheses constructed in accordance with the teachings ofthe present invention. FIG. 30 illustrates a superior vertebral body1010, an inferior vertebral body 1020 and a functional spinal unit 1000(i.e., a pair of facet joints) between the bodies 1010, 1020. In FIG.31, a portion of the superior vertebral body 1010 and the functionalspinal unit 1000 has been removed during a decompressive laminectomyprocedure. Depending upon the surgical need, some or all of theposterior portions of one or more vertebral bodies may be damaged orremoved, or the area may be weakened by degeneration and/or disease,rendering them unsuitable for anchoring a portion of the prosthesis intoand/or through the lamina.

In the event of weakened posterior portions, embodiments of the presentinvention may be adapted to attach to different portions of thevertebral body that remain structurally sound. For example, in theillustrated embodiment of FIG. 32 a portion of the prosthesis, in thisembodiment the caudal portion 1030 of the prosthesis, can be anchoredinto the lamina of the inferior vertebral level 1030. However, thesuperior vertebral body 1010 has a compromised laminar structure. Assuch, the other portion of the prosthesis, here the cephalad portion1040 in FIG. 33, is anchored into the pedicles of the superior vertebralbody 1010. Desirably, the cephalad and caudal portions 1040 and 1030articulate with respect to each other through the interactions of one ormore bearing surfaces. For example, FIG. 33 illustrates an embodimentwhere the caudal prosthesis 1030 has a bearing surface 1035 thatinteracts with the cephalad bearing 1045. The bearing surface 1035 is onan anterior surface of the caudal prosthesis 1030 in the illustratedview. FIG. 34 illustrates another alternative embodiment where thecaudal bearing surface 1036 is on a posterior surface of the caudalprosthesis 1030 to interact with the cephalad bearing 1045.

In another alternate embodiment, both the cephalad and caudal portionsof the prosthesis could be secured to the lamina and/or spinous processof their respective vertebral bodies using embodiments of the presentinvention described above with regard to FIGS. 15-28. Advantageouslyutilizing the lamina and/or spinous process of each vertebra preservesthe pedicles of both the cephalad and caudal vertebral levels for futuretreatment, permit treatment of a level involving previously-existingpedicular fixation, and/or allow treatment of an unfused level boundedby one or more upper and/or lower adjacent fused levels.

FIG. 35 illustrates other embodiments of prostheses 200 in place on avertebral body 14. FIG. 35 illustrates reinforcing materials 3500 and3505 disposed between collars 225 and 220 and the vertebral bone. Inboth embodiments, the reinforcing material 3500 and 3505 is desirably“wider” than the adjacent collars and assists in the distribution of thecompressive load from the collars to a larger area of the vertebra.These reinforcing materials are formed from any suitable material todistribute such loading, and may be malleable enough to be formed aroundand contour with the bone (while strong enough to distribute compressiveloads), semi-malleable or rigid (and may comprise one or more series ofdifferent size and/or shape devices that correspond to varying anatomyof the targeted vertebral body). In various embodiments, the reinforcingmaterial can be formed from or is coated with a biocompatible coating orhas a surface in contact with bone that is treated with compounds topromote adhesion to the bone or bone growth.

In one embodiment, the reinforcing material is a strip 3500 desirablyhaving sufficient length to extend from the proximal collar 220, alongthe bone to the distal collar 225. In another alternative embodiment,the reinforcing material is one or more pads having a larger area thanthe adjacent collar. In this case, the reinforcing material 3505 isshown adjacent a distal collar 225 (a similar reinforcing material maybe placed adjacent the opposite proximal collar, if desired, which ishidden by the lamina in this figure. In additional alternativeembodiments, the reinforcing material could be used in conjunction withor in lieu of one or both collars 225, 220.

The various disclosed embodiments can facilitate the implantation of oneor more portions of a facet joint prosthesis into a vertebral levelthat, for a myriad of reasons, cannot accommodate a pedicle-fixationbased anchor. Such reasons can include where the pedicular space isalready occupied (i.e., where pedicle screws are being used in fusionsystems and/or dynamic stabilization), is blocked (i.e. where existinghooks and/or rods block clear access to the pedicle), or where thevertebral body is partially or fully occupied to preclude introductionof pedicle anchors (i.e. where fixation screws for thoraco-lumbar platespass in front of the pedicle space, or the vertebral body incorporatesinterbody spacers such as in the K-Centrum Anterior Spinal System™).

It should be understood that the caudal and cephalad bearing surfaces inother embodiments could be reversed, such that the bearing surface ofthe caudal component could ride inside and/or against a bearing surfaceof the cephalad component. For example, one or more balls (incorporatingconvex bearing surfaces) carried by the caudal components could rideinside one or more corresponding cups (incorporating concave bearingsurfaces) carried by the cephalad components. In a similar manner, thearms of the caudal components could be longer than the arms of thecorresponding cephalad components.

In various alternate embodiments, the disclosed pairs of caudal andcephalad portions of the facet replacement prosthesis described hereincould comprise a single caudal and cephalad pair, or more than twocaudal and cephalad pairs. Alternatively, the disclosed caudal paircould comprise a single caudal portion, with one or a plurality ofcephalad portions, and vice versa. In such an arrangement, the pluralityof portions could interact with a single bearing surface on thecorresponding single portion, or multiple bearing surfaces on thecorresponding single portion of the prosthesis.

While the disclosed bearing surface 235 a is spherical in shape, thebearing surface 235 a could be a myriad of shapes, including othergeometric configurations as well as flat, convex or concave surfaces. Inone embodiment, the bearing surface could comprise a relatively flatsurface having a convex face, similar to the surface of the facet jointit replaces.

The above described embodiments of this invention are merely descriptiveof its principles and are not to be limited. The scope of this inventioninstead shall be determined from the scope of the following claims,including their equivalents.

1. A method of stabilizing a spinal motion segment, comprising:accessing a facet joint of the spinal motion segment; resecting all or aportion of the facet joint to provide a surgical access path through atleast a portion of the resected all or a portion of the facet joint forthe implantation of a first motion-allowing implant within anintervertebral disk space of the spinal motion segment; and introducingthe first motion-allowing implant into the intervertebral disk spacethrough the surgical access path.
 2. The method of claim 1, wherein thefirst motion-allowing implant comprises an artificial disk.
 3. Themethod of claim 1, wherein the first motion-allowing implant comprises adisk augmentation device.
 4. The method of claim 1, further comprisingthe step of implanting a second motion-allowing implant to replacesubstantially all of the resected all or a portion of the facet joint.5. The method of claim 1, further comprising the step of implanting asecond motion-allowing implant to replace substantially all of thefunction of the resected all or a portion of the facet joint.
 6. Themethod of claim 4, wherein the second motion-allowing implant comprisesa facet replacement device.
 7. The method of claim 4, wherein the stepof implanting the second motion-allowing implant occurs after the stepof introducing the first motion-allowing implant through the surgicalaccess path.
 8. The method of claim 1, wherein the resecting stepcomprises resecting substantially all of the superior facet and theinferior facet of the facet joint.
 9. The method of claim 8, wherein theresecting step further comprises resecting substantially all of thesuperior and inferior facets of a second facet joint of the spinalmotion segment.
 10. The method of claim 1, wherein the resecting stepfurther comprises resecting a portion of the lamina of the spinal level.11. The method of claim 5, wherein substantially all of the surgicalsteps occur percutaneously.
 12. The method of claim 5, whereinsubstantially all of the surgical steps are performed using acombination of open and percutaneous surgical accesses.
 13. The methodof claim 5, wherein at least one of the surgical steps is performedthrough a cannula.
 14. The method of claim 13, wherein at least one ofthe surgical steps is performed through an expanding cannula.
 15. Themethod of claim 4, wherein the second motion-allowing implant isattached to one or more of the vertebral bodies of the spinal motionsegment.
 16. The method of claim 4, wherein the second motion-allowingimplant is attached to one or more pedicles of a vertebral body of thespinal motion segment.
 17. The method of claim 4, wherein the secondmotion-allowing implant is attached to one or more lamina of a vertebralbody of the spinal motion segment.
 18. The method of claim 4, whereinthe second motion-allowing implant is attached to one or more spinousprocesses of a vertebral body of the spinal motion segment.
 19. Themethod of claim 4, wherein the biomechanics of the secondmotion-allowing implant is designed to substantially mimic the naturalmotion and flexibility of the facet joint.
 20. The method of claim 4,wherein the biomechanics of the second motion-allowing implant isdesigned to a lesser degree of flexibility and/or motion than allowed bythe facet joint.
 21. The method of claim 4, wherein the biomechanics ofthe second motion-allowing implant is designed to a greater degree offlexibility and/or motion than allowed by the facet joint.
 22. Themethod of claim 4, wherein the biomechanics of the secondmotion-allowing implant is designed to a substantially mimic the naturalmotion and flexibility of the facet joint and any resected or surgicallydisrupted anatomical structures